Q: Engine caps at 3000 rpm's, no cel light, and no smoke of any color. Uncaps when restart the engine.

I recently found out that my Mazda 3 sport TS2 Diesel 1.6l is capping the revs at 3000 at some points, I am not too sure but I think it happens when going uphill (slight not steep).

I experiences a bang while on the highway and loss of power but car kept on going, no smoke, no cel light (the led works). It had a hard time getting back home, struggling to make it past 60mph foot full on gas pedal etc. Next day car was fine, revved up above 3k, driving normal. Until it caps again, but doesnt struggle as much. In fact it behaves normally until you want to go on higher gears (4rth and above when it's capped, when it's not yet capped it goes like an arrow). THe cel light has never gone off, no smoke all over the place white or black.

Reset the ECU, Cleaned the MAF sensor and the car seemed like a totally different car, going super smooth. After some hours the car feels almost like preclean again. No idea what could be wrong

My car has 21000 miles.
My car has a manual transmission.